PESQ (Perceptual Evaluation of Speech Quality) and its successor POLQA (Perceptual Objective Listening Quality Assessment) are full-reference measures described in ITU standards that allow for prediction of speech quality by comparing a reference signal to a received signal. However, clock drift is a commonly encountered problem in many systems (e.g., VoIP systems), and can cause a drop in speech quality estimates from PESQ or POLQA.
While there may be a range of QoS (Quality of Service) metrics available to predict delay and clock drift, such metrics are limited in their abilities to predict the end-user perceptual quality of experience.